Electric stimulation of tissue is used to treat a variety of disorders. Specifically, implantable electric stimulators and leads have been used to treat chronic pain, muscular disorders, hearing problems, symptoms of Parkinson's Disease, bladder control, and sexual dysfunction, among others. Often, a lead terminating in electrodes is situated close to a tissue such as spinal cord, nerve roots, muscles, or brain tissue. An implanted signal generator connected to the lead is then used to generate patterns of electric pulses that stimulate the tissue.
For example, electrodes may be placed in proximity to the spinal cord to treat a variety of ailments. Depending on the placement, electric stimulation will affect various dermatomes about the body. Chronic pain about the leg might be managed by stimulating the spinal cord between the T11 vertebra and the L3 vertebra. Symptoms of Parkinson's Disease and Epilepsy may be treated by placing electrodes about the sub-thalamic region of the brain. In addition, other ailments such as muscular spacticity, incontinence, and sexual dysfunction may be treated by pulse stimulation of an associated tissue.
One typical type of lead is a percutaneous lead. These leads are often designed for easy introduction into an epidural space about the spinal cord using a special needle. As such, these leads are typically cylindrical and terminate at a distal end in one or more cylindrical electrodes distributed longitudinally along the percutaneous lead. A stimulation pulse is typically established by providing opposite charges to one or more electrodes along the lead or between electrodes on adjacent leads. Various pulse amplitudes, pulse widths, frequencies, and intervals, may be used with varying effect on the desired tissue.
However, the stimulation pulses may also affect other tissue in the region. Since the leads are cylindrical, they initiate a charge flow field 360 degrees about the electrode. In many applications where the desired stimulation region is small, the large flow field created by these electrodes stimulates other tissue, having undesired effects.
Controlled tissue stimulation is especially important in deep brain stimulation. The brain is a compact organ in which many differing functions occur in close proximity. Broad stimulation of a small region about the ventrointermediate nucleus of the thalamus could have the positive effect of reducing tremors associated with Parkinson's Disease or cause unwanted side effects such as paresthesia about the head and hands, depression, peresis, dysarthia, loss of balance, or dystonia. Often, patients undergo expensive and painful surgery only to discover that the side effects are to great to continue treatment.
Another area of concern is near the terminal end of the spinal cord. A large number of nerve roots extend from a relatively small region of the spinal cord. Excess stimulation could produce paresthesia in an undesired dermatome or inhibit muscular response in another region.
Another type of lead is paddle-like leads in which electrodes are placed on one side of a flat surface. These electrodes have the disadvantage that they only stimulate in one direction and do not have the option of stimulating in other directions. Misplacement of the electrode can only be corrected by surgery. Often, these electrodes can only establish flow fields longitudinally along the one direction as well. The longitudinal spacing is often large, also causing the stimulation of extra tissue bringing about an undesirable effect.
As such, many leads in electrode configurations suffer from deficiencies caused by the stimulation of extra tissue bringing about undesirable effects. Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.